Hybrid vehicles require accurate control of the electric motor in order to achieve maximal fuel savings while ensuring good driveability and safety. The most prevalent control method for advanced traction motors is “field oriented control” (FOC). In FOC, the three-phase current wave-forms and voltage wave-forms (fixed frame) are transformed into a two-axis dq-frame which is rotating at the frequency ωs of the electrical waveforms (synchronous frame). By this coordinate transformation, a.c. waveforms result in d.c. vectors (also called space vectors). The advantage of this approach is that it is much easier to control d.c. quantities than a.c. quantities. The implementation of a digital current regulator is therefore relatively straightforward and can be very robust and dynamic.
Typically a field-oriented motor drive has a three-phase inverter connected on the d.c. side to an energy storage device (such as a battery) and on the a.c. side to an electric motor. The six switches (e.g. IGBTs or MOSFETs) are controlled by a pulse-width modulation (PWM) module, which, in turn, is commanded according to the output of a synchronous frame current regulator. The inputs to the synchronous frame regulator method include the following:
lu, Iv: Phase current measurements
Id*, Iq*: Direct and quadrature current setpoints (calculated by the higher level motor control algorithms)
θr: Rotor flux angle (determined by a rotor flux estimator).
One of the keys to an accurate motor control is accurate current control, which in turn requires accurate measurement of the motor current. The three phase output of a traction drive is typically only instrumented with current sensors on two phases. The reason for this is that the third current can be calculated from the measurement of the other two, based on Kirchoff's law that the sum of current flowing into a node (the motor) must be equal to zero, i.e.Iw=−(Iu+Iv)
Several types of sensors can be used for measuring inverter currents. A discussion of the different technologies and advantages/disadvantages can be found in CURRENT SENSING IN ELECTRICAL DRIVES—A FUTURE BASED ON MULTIPLE INNOVATIONS by Eric Favre, Wolfram Teppan, LEM Group, herein incorporated in its entirety by this reference. Because traction drives operate down to very low frequencies including 0 Hz, the current sensors in a traction inverter must be capable of measuring d.c. and a.c. currents. Furthermore, for adequate control response, the sensors need to have a bandwidth of several kHz, typically 50-100 kHz. Hall-effect based current sensors are well suited for measuring current over a wide frequency range. Two types of such sensors exist: “closed-loop” and “open-loop”. Both types are based on an arrangement in which the conductor carrying the current to be measured is routed through a gapped core. Located in the airgap is a Hall-effect sensor which measures the flux in the core.
In the open-loop approach, the flux in the core is solely induced by the current carrying conductor and the output of the Hall sensor is used directly as the current measurement. Due to gain tolerances of the Hall sensor, tolerances on core material properties, and variations in the mechanical positioning of the sensor in the airgap, open-loop sensors are not very accurate (typically within 5-10% of rated output).
Closed-loop Hall sensors achieve a significant improvement in accuracy by using a compensation-coil wound on the sensor core and supplied such as to cancel the flux in the core. The Hall sensor acts as a feedback for the flux-canceling loop and is not directly used as a current measurement. Instead, the current in the compensating winding serves as the measurement output and is typically converted into a voltage by means of a resistive shunt. Closed-loop sensors can achieve accuracies that are better than 1%. While closed-loop sensors offer advantages in terms of accuracy, they also have some significant drawbacks. First and most importantly, their power-consumption is proportional to the measured current and can be quite large (several watts). Furthermore, they need to be supplied by a dual supply of +/−12V or higher. In contrast, open-loop sensors can operate from a single supply as low as 5V and consume fractions of one watt of power. This difference is important, because power-supply requirements have a significant impact on the overall cost of the inverter. Closed-loop sensors also tend to be larger and more expensive than open-loop sensors. Those drawbacks are particularly penalizing for larger inverters, with output current exceeding 200-300 amps. Since traction inverters are very cost sensitive, using closed-loop sensors in such applications is often not a viable option, and open-loop sensors are used instead. However, if the gain-error of the sensors is not compensated for, the performance of the drive will suffer. Besides torque linearity problems, gain-errors can also cause torque ripple and drive-train oscillations.
This means that some method of calibrating the open-loop sensors must be used in order to achieve acceptable drive performance. A factory calibration of each individual sensor could be performed during the manufacturing process. However, this operation is expensive and carries the overhead of handling individual calibration constants. Furthermore, this approach can not address gain variations over time or drift with temperature for units in the field.
This problem has been recognized by Ford Motor Company, who proposed a solution in U.S. Pat. No. 6,998,811, entitled, COMPENSATION METHOD FOR CURRENT-SENSOR GAIN ERRORS, Feb. 14, 2006, by Myers et al. herein incorporated in its entirety by this reference, which is based on injecting a high frequency carrier signal electrically into the electric motor and using the high-frequency negative sequence of the measured current to adjust the sensor gains in a closed-loop fashion.
There are apparently several issues with this approach. The injected carrier needs to be at a frequency that is substantially higher than the fundamental frequency of the currents; this increases the bandwidth requirements of the current sensors and therefore their cost, especially for high-speed or high-pole motors. The injected high-frequency carrier can result in emissions that interfere with other circuits or devices in the system. Operation can be affected by saliencies or imbalances in the electric motor; drivetrain oscillations; and noise on the current measurements. It is also computationally intensive, requiring an additional sine/cosine calculation for the negative sequence measurement as well as higher-order filters to extract the carrier signal. The absolute accuracy of the approach depends on the “theoretical unit vector” input, which needs to be model-based and therefore is vulnerable to variations and tolerances of the electric motor. It needs to be tuned and verified for a given electric motor type.
Another approach for dealing with the inaccuracies of open-loop Hall sensors is to pair them with current transformers (CT). For example, U.S. Pat. No. 5,479,095, Dec. 26, 1995 entitled METHOD AND APPARATUS FOR MEASURING AC AND DC CURRENT by Michalek et al. herein incorporated in its entirety by this reference, which consists of using two sensors (one Hall, one CT) on the same conductor to measure the conductor-current. Based on a threshold decision, either the output of the Hall sensor or the output of the current transformer is used as the more accurate current measurement. U.S. Pat. No. 5,146,156, Sep. 18, 1992, entitled CURRENT INTENSITY TRANSFORMER DEVICE FOR MEASURING A VARIABLE ELECTRIC CURRENT by Etter Marcel herein incorporated in its entirety by this reference, proposes an integrated sensor consisting of both a Hall device and a sensing coil (analogous to a current transformer). Both measurement outputs are added together with a frequency dependent weight, resulting in a measurement output at higher current frequencies that is superior in accuracy to the output of the Hall sensor alone. Both these approaches suffer from the following drawbacks. They are not cost-effective in a three-phase system, since two sensing techniques are used on one and the same connector. They do not “learn”, i.e. the additional measurement accuracy provided by the CT is only available at higher frequency currents, but is not being used to calibrate the Hall sensor for improved low frequency measurements.